Light emitting diodes (LEDs) often are used as light sources in liquid crystal displays (LCDs) and other displays. The LEDs often are arranged in parallel “strings” driven by a shared voltage source, each LED string having a plurality of LEDs connected in series. To provide consistent light output between the LED strings, each LED string typically is driven at a regulated current that is substantially equal among all of the LED strings.
Although driven by currents of equal magnitude, there often is considerable variation in the bias voltages needed to drive each LED string due to variations in the static forward-voltage drops of individual LEDs resulting from process variations in the fabrication and manufacturing of the LEDs. Dynamic variations due to changes in temperature when the LEDs are enabled and disabled also can contribute to the variation in bias voltages needed to drive the LED strings with a fixed current. In view of this variation, conventional LED drivers typically provide a fixed voltage that is sufficiently higher than an expected worst-case bias drop so as to ensure sufficient voltage headroom at the current regulators that control the currents through the LED strings. However, as the power consumed by the LED driver and the LED strings is a product of the output voltage of the LED driver and the sum of the currents of the individual LED strings, the use of an excessively high output voltage by the LED driver unnecessarily increases power consumption by the LED driver. Moreover, the operation of the components of the LED driver itself can lead to excessive power consumption.